Offset GPS altitude metadata

[Lùrenz]

Hi everybody, i would like to ask some advice on how to change some specific data (GPS altitude and altitude reference) in a set of pictures took with my dji drone, if it's possible I would prefer to offset all those values to obtain a quite precise elevation instead of setting the same altitude for all images to prevent the flattening of the entire dataset.

[Phil Harvey]

For example, to subtract 20.5 meters:

exiftool -gpsaltitude+=-20.5 DIR

- Phil

[StarGeek]

The relevant part of the documentation under the -TAG[+-^]=[VALUE] option

+= may also be used to increment numerical values (or decrement if VALUE is negative),

Note that to subtract, you have to "add" a negative number as in Phil's example.  That's because -= is a different operation. Directly after the above documentation is

and -= may be used to conditionally delete or replace a tag

[Lùrenz]

Thank you!